1. Field of the Invention
The invention relates in general to an apparatus and method for manufacturing plugs, and more particularly to an apparatus and method of manufacturing plugs for a blind via and a micro via and an image alignment apparatus.
2. Description of the Related Art
High-Density Interconnection (HDI) technology is usually required to form a micro via and a blind via. These vias are formed by several advanced technologies, including Laser Ablation, Plasma Etching, Photo Imageable Dielectric (PID) films and Mechanical Drilling. The typical size of these via are around 2.about.6 mils in diameter. They are widely used in Printed Circuit Boards (PCB), Multi-Chip Module-Laminates (MCM-L), ceramic substrates, thick film devices and Integrated Circuit Packaging Carriers such as Ball Grid Arrays (BGA), Chip Scale Packages (CSP) and so on.
Solid masks (S/M), epoxies, or conductive pastes, such as silver, copper or silver-copper mixed pastes, are commonly used to plug these micro vias and blind vias. This process is known as a plug process. Because the micro vias are very small, it is very hard to form bubble-free plug.
A stencil is aligned to the board by using the conventional stencil printer process. The conventional printer simply applies a lot of paste on the stencil. A squeegee is then used to plug these vias with paste.
However, the typical size of these vias is around 2.about.6 mils in diameter. By using the conventional Surface Mount Technology (SMT) screen printer or stencil printer to plug these vias, a problem of voids arises. The voids produced during the printing process are harmful to the reliability test. Moreover, entrapped bubbles can be formed in the paste and voids can be formed in the plug during the printing processes due to exposure to the atmosphere.
The paste usually consists of flux and solvent and its viscosity continues to change as the solvent evaporates during the printing processes. The conventional printer simple applies the paste onto the stencil exposed to the air, and then mixes the paste. Therefore, bubbles can be trapped in the paste during the mixing step. It is easy to form voids in the plug. Meanwhile, the solvent evaporates, accompanied by changes in the paste's viscosity. So there is no way to keep a constant quality of paste during the conventional process.
The precise alignment between the stencil and the PCB is not achieved by conventional process due to the small dimension of the via. The conventional method fixes the stencil first. The cameras are aligned to the stencil by the fiducial mask on the stencil. The cameras are then moved out of the printing region. The board is aligned to the cameras and then moved into the printing area. In most conventional printers, the board has to move to the printing area after each alignment which may introduce and acaccumulate alignment inaccuracies. Therefore, at least one re-alignment step is performed after several printing processes.